The present invention relates to a method for the removal of one or more contaminants selected from the group consisting of sulfur dioxide (“SO2”) and NOx from gaseous carbon dioxide. The invention has particular application in the purification of carbon dioxide flue gas from an oxyfuel combustion process, for example, in a pulverized coal fired power station in which sulfur containing carbonaceous or hydrocarbon fuel is combusted in a boiler to produce steam for electric power generation.
The term “NOx” means at least one nitrogen oxide compound selected from the group consisting of nitric oxide (“NO”) and nitrogen dioxide (“NO2”).
It has been established that one of the main causes of global warming is the rise in greenhouse gas contamination in the atmosphere due to anthropological effects. The main greenhouse gas which is being emitted, carbon dioxide (CO2), has risen in concentration in the atmosphere from 270 ppm before the industrial revolution to the current figure of about 378 ppm. Further rises in CO2 concentration are inevitable until CO2 emissions are curbed. The main sources of CO2 emission are fossil fuel fired electric power stations and from petroleum fuelled vehicles.
The use of fossil fuels is necessary in order to continue to produce the quantities of electric power that nations require to sustain their economies and lifestyles. There is, therefore, a need to devise efficient means by which CO2 may be captured from power stations burning fossil fuel so that it can be stored rather than being vented into the atmosphere. Storage may be in a geological formation such as a saline aquifier or a depleted oil or natural gas formation. Alternatively, the CO2 could be used for enhanced oil recovery.
The oxyfuel combustion process seeks to mitigate the harmful effects of CO2 emissions by producing a net combustion product gas consisting of CO2 and water vapour by combusting a carbonaceous or hydrocarbon fuel in pure oxygen. This process would result in an absence of nitrogen in the flue gas, together with a very high combustion temperature which would not be practical in a furnace or boiler. In order to moderate the combustion temperature, part of the total flue gas stream is recycled, after cooling, back to the burner.
An oxyfuel process for CO2 capture from a pulverised coal-fired power boiler is described in a paper entitled “Oxy-combustion processes for CO2 capture from advanced supercritical PF and NGCC power plants” (Dillon et al; presented at GHGT-7, Vancouver, September 2004), the disclosure of which is incorporated herein by reference.
Oxyfuel combustion produces a raw CO2 product containing contaminants such as water vapour, “inerts” including excess combustion molecular oxygen (O2), molecular nitrogen (N2) and argon (Ar) derived from the oxygen used, any air leakage into the system, and acid gases such as sulfur trioxide (SO3), sulfur dioxide (SO2), hydrogen chloride (HCl), nitric oxide (NO) and nitrogen dioxide (NO2) produced as oxidation products from components in the fuel or by combination of N2 and O2 at high temperature. The precise concentrations of the gaseous impurities present in the flue gas depend on the fuel composition, the level of N2 in the combustor, the combustion temperature and the design of the burner and furnace.
In general, the final CO2 product will be produced as a high pressure fluid stream for delivery into a pipeline for disposal. The CO2 must be dry to avoid corrosion of the carbon steel pipeline. The CO2 impurity levels must not jeopardise the integrity of the geological storage site, particularly if the CO2 is to be used for enhanced oil recovery, and the transportation and disposal must not infringe international and national treaties and regulations governing the transport and disposal of gas streams.
It is, therefore, necessary to purify the impure CO2 from the boiler or furnace to remove water vapour, sulfur trioxide and sulfur dioxide (“SOx”), nitric oxide and nitrogen dioxide (“NOx”), soluble gaseous impurities such as HCl, and “inert” gases such as O2, N2 and Ar in order to produce a final CO2 product which will be suitable for disposal.
In general, the prior art in the area of CO2 capture using the oxyfuel process has up to now concentrated on removal of SOx and NOx upstream of the CO2 compression system using current state of the art technology. SOx/NOx removal is based on flue gas desulphurisation schemes such as scrubbing with limestone slurry followed by air oxidation producing gypsum and NOx reduction using a variety of techniques such as low NOx burners, over firing or using reducing agents such as ammonia or urea at elevated temperature with or without catalysts. Conventional SOx/NOx removal using desulphurisation and NOx reduction technologies is disclosed in “Oxyfuel Combustion For Coal-Fired Power Generation With CO2 Capture—Opportunities And Challenges” (Jordal et al; GHGT-7, Vancouver, 2004). Such process could be applied to conventional coal boilers.
A process for the conversion of SOx/NOx, present in the stack gas of fossil fuel fired boilers, into concentrated H2SO4 and HNO3 has been developed Tyco Labs., Inc. and is described in a report titled “Development of the catalytic chamber process for the manufacture of sulphuric and nitric acids from waste flue gases” (Keilin et al; Contract number PH86-68-75; Prepared for the US Environmental Protection Agency Office of Air Programs 1967 to 1969). The Tyco process is based on the lead chamber process for sulphuric acid manufacture. In this process SO2 is oxidized to SO3 by reaction with NO2 (see Equation (a));SO2+NO2=SO3+NO  (a).This reaction is followed by dissolution of the SO3 in water to form sulphuric acid (see Equation (b));SO3+H2O=H2SO4  (b).The NO is reoxidized to NO2 by reaction with oxygen present in the flue gas (see Equation (c));2NO+O2=2NO2  (c)The NOx acts as a gas phase catalyst.
This process would not normally be feasible at atmospheric pressure and with the low concentrations of NOx present.
A further problem would be the rather slow kinetics of the NO oxidation step. The Tyco process gets over this problem in two ways. First, it increases the NO2 concentration in the stack gas by a factor of about 100 by recycling an NO2 rich gas stream which is mixed with the stack gas prior to SO2 oxidation and H2SO4 production. The H2SO4 is recovered in a high temperature scrubber, which allows the bulk of the water vapour in the stack gas to pass through the unit without condensation, producing an acid of about 80% concentration. The NO2 and NO react with the sulphuric acid to form nitrosyl sulphuric acid so that about 90% of the NOx present in the flue gas is removed together with virtually all of the SOx (see Equation (d)).NO2+NO+2H2SO4=2NOSO4+H2O  (d).
Secondly, the slow oxidation of NO to NO2 is speeded up by passing the nitrosyl sulphuric acid through a stripper tower which is swept by a small side-stream of the flue gas feed which provides the O2 needed for net NO oxidation to NO2. The oxidation reaction in the stripper tower is assisted by an active carbon catalyst which circulates in the liquid phase.
There is a need for an improved method for the removal of SOx/NOx from gaseous carbon dioxide, particularly from carbon dioxide flue gas produced in an oxyfuel combustion process such as that involved in a pulverized coal-fired power boiler.